When there is no particular problem in fixing the module to the base of the timepiece movement, particularly to the bottom plate, by a vertical translation which enables a first wheel set of the module to mesh with a second wheel set pivoting in the base, the module is generally positioned by two feet (or pins) machined or arranged in the base, which penetrate two respective holes machined in a plate or bar of the module. Once positioned, the module is secured by at least one screw to the base. Given the manufacturing tolerances for the feet, holes and bearings of the first and second wheel sets in various parts of the ébauche, the positioning of the module is approximate and there is a significant variation in the distance of centres of the first and second wheel sets. This gives rise to a real problem for the rate of the timepiece movement, since, if the gearing of the first and second wheel sets is not optimal, efficiency is reduced and becomes variable. The amplitude of the balance is generally reduced and this affects the rate accuracy of the timepiece. In this respect, accurate and well defined meshing between the gear train and the platform escapement exit is particularly important.
It is not generally possible to position the module by a simple vertical translation. One solution to this particular problem is given in the following Patent documents: CH-578203/U.S. Pat. No. 3,802,183 and CH-581342/U.S. Pat. No. 3,945,197. These documents propose assembling a platform escapement by performing a vertical translation to arrange a guide hole in the platform escapement on one foot of the bottom plate (an internal threaded cylinder or pin for a fixing screw) about which it can rotate. Next, the platform escapement is rotated until two respective parts of the platform escapement and the rest of the movement are abutting against each other. In the first document, a column (pin) of the platform escapement abuts against a side wall of a bar in which the fourth wheel set pivots. The escape pinion must mesh with the fourth wheel set. In the second document, one area of a plate of the platform escapement abuts against a bar mounted on the bottom plate. In both cases, the problem of various manufacturing tolerances which vary the distance of centres of the escape pinion and the fourth wheel remains unchanged.
FIGS. 1 to 3 show a movement in which a platform escapement 2 is mounted on a bottom plate 4 in accordance with the aforementioned technique. Escape wheel set 6 is pivotally mounted between a lower plate 8 and a top bar 10 of the platform escapement, as is the balance 12. In FIG. 1, the platform escapement is mounted on a foot or, as shown, on an internally threaded cylinder 16, about which it can turn to be brought, via a horizontal rotation, into a final position shown in FIGS. 2 and 3. In this final position, pinion 18 of the escape wheel set meshes with a gear train wheel 14 mounted on the bottom plate. To hold the platform escapement in its final position, two screws are provided; the first is screwed into cylinder 16 and the second is screwed into a threaded hole 20 in the bottom plate through a truncated hole 22 in top bar 10. The assembly tolerances of the escape wheel set and wheel 14 in the platform escapement and on the bottom plate respectively mean that it is not possible to obtain a precise, predetermined distance of centres. Moreover, the assembly of the platform escapement on the bottom plate has a large tolerance which considerably increases the problem. Thus, the penetration of the toothing of pinion 18 in the toothing of wheel 14 is not precisely determined and has a variable, uncontrolled nature, which has a negative influence on the operation of the escapement.
Generally, when a module, which forms a distinct unit with at least a first wheel set pivoting therein, is arranged on a timepiece movement base and the first wheel set has to mesh with a second wheel set pivoting on the base, there is a problem linked to the fact that the distance of centres of the first and second wheel sets varies around the optimum distance of centres; which affects the proper operation of the timepiece movement. Indeed, the gearing between the first and second wheel sets is made worse by numerous manufacturing tolerances; particularly with the modular construction provided here. It is therefore difficult to obtain a predetermined distance of centres.